The present invention relates to cryogenic refrigeration systems, and in particular to an expansion engine used in refrigeration and liquefaction cycles to produce low temperature gases and liquids.
To achieve liquid helium temperatures for applications involving superconducting magnet and solenoid devices, industry has a serious need for highly reliable, high efficiency refrigerator/liquefiers using expansion engines to produce equivalent capacity ranges of 5 to 15 liters per hour of liquid helium.
Among the emerging industrial uses for superconducting components are superconducting magnetic energy storage (SMES) systems which may be used to store and instantaneously provide electrical power to offset damaging voltage dips caused by routine circuit-switching at power substations. Although lasting a fraction of a second, voltage dips can cause significant damage to electronic controllers essential to manufacturing operations, and uninterrupted power sources are thus critical to prevent idling entire manufacturing plants. A single downtime incident in a large manufacturing plant can result in hundreds of thousands of dollars in losses. Total losses in the United States due to voltage dips have been estimated to be more than $12 billion dollars annually.
Essential to commercial viability of SMES is a reliable cryogenic refrigeration/liquefaction system which can supply needed refrigeration at liquid helium temperatures to sustain the superconducting devices therein. Commercially available refrigeration/liquefaction systems providing capacity needed for micro and mini SMES applications exhibit a predictable 3,000 to 4,000 hour mean time between failure rate (4 to 51/2 months). Failure is generally attributed to the failure of expansion engines employed in these systems. Sources of expansion engine failure include cryogen leakage, air and oil contamination of cryogens, excessive wear, and fatigue. Commercial acceptance of SMES systems, which function as back-up safety systems, requires that they achieve much higher levels of reliability.
Further, because of the cryogenic temperatures at which expansion engines must operate in such refrigeration/liquefaction systems, low thermal losses and high thermal efficiency are at a premium. Commercially available expansion engines currently exhibit lower thermal efficiencies and higher thermal losses than are desirable.
Accordingly, the need exists for high reliability expansion engines which operate with extended mean times between servicing or failure, with low thermal losses and high efficiencies, to satisfy the demands of various existing and emerging applications involving superconducting devices.